Alterations in protein expression levels are mediated by a number of mechanisms including the up- and down- regulation of the transcription of genes in response to different external stimuli such as nutrient conditions. In addition, these cells can also stimulate the production or repression of protein levels by modulating post-transcriptional processes such as translation. For this reason, the sole analysis of the levels of mRNA expression does not sufficiently represent the changes that occur at the protein level in the cell. Previous data has indicated that in S. cerevisiae, the levels of a number of transcriptional regulatory proteins including some members of mRNA elongation complexes are regulated by post-transcriptional mechanisms in response to growth in minimal media. We hypothesize that these changes in protein content lead to alterations in either the composition or stoichiometry of the protein complexes involved in the regulation of transcriptional elongation as well as the regulation of other nuclear processes. Using a quantitative proteomics approach, we will determine how changes in the level of protein expression alter the makeup of protein complexes and analyze how these changes alter the gene expression profile of the cell.